The phase current in each phase of a multi-phase step motor must be carefully controlled to insure precise motor rotation and to hold the motor in a fixed position upon stoppage. Methods for such motor control usually include a simple comparator and a set point for determining when the motor phase current is at or above the set point.
However, at low motor currents, commutation transients can cause the comparator to prematurely indicate that the set current has been reached and audible noise is often generated due to the uncertainty of the comparator switch point.
U.S. Pat. No. 5,650,705 entitled “Apparatus and Method for Controlling Currents in an Inductor”, U.S. Pat. No. 5,264,770 entitled “Stepper Motor Driver Circuit” and U.S. Pat. No. 6,049,184 entitled “Method and Arrangement for Controlling A Current” each describe various methods for accurately controlling current in stepper motor applications.
One method used to eliminate the noise effects during switching transients consists of blocking or “masking” the sense signal during the transient. When at low current settings, simply masking the sense signal can cause errors if the motor current increases above the set point during the mask period, as a result, the phase current can be, on average, too high. Not masking commutation transients can cause the phase current to be shut off before the set point is reached; as a result, the phase current can be, on average, too low. Further, audible noise is often generated due to the uncertainty of the switch point.
One example of a mask control signal used in controlling a stepper motor is described within U.S. Pat. No. 5,003,261 entitled “Rotation Detection Circuit Having Input Pulse Signals for a Step Motor”.
One purpose of the present invention is to improve stepper motor accuracy at low phase current values.